To enhance the performance of metal-oxide-semiconductor (MOS) devices, stress may be introduced in the channel regions of the MOS devices to improve carrier mobility. Generally, it is desirable to induce a tensile stress in the channel region of an n-type MOS (“NMOS”) device in a source-to-drain direction, and to induce a compressive stress in the channel region of a p-type MOS (“PMOS”) device in a source-to-drain direction.
A commonly used method for applying compressive stress to the channel regions of PMOS devices is growing SiGe stressors in the source and drain regions. Such a method typically includes the steps of forming a gate stack on a silicon substrate, forming spacers on sidewalls of the gate stack, forming recesses in the silicon substrate and adjacent the gate spacers, and epitaxially growing SiGe stressors in the recesses. An annealing is then performed. Since SiGe has a greater lattice constant than silicon, it expands after annealing and applies a compressive stress to the channel region of the respective MOS device, which is located between a source SiGe stressor and a drain SiGe stressor.
A chip may have different regions having different pattern densities. Due to the pattern loading effect, the growth of SiGe stressors in different regions may have different rates. For example, FIG. 1 illustrates the formation of SiGe regions for PMOS devices in logic device region 300 and static random access memory (SRAM) region 400. Since the pattern density of the PMOS devices in SRAM region 400 is generally higher than the pattern density of the PMOS devices in logic region 300, and the sizes of SiGe regions 410 are typically smaller than the sizes of SiGe regions 310, SiGe regions 410 are grown faster than SiGe regions 310. As a result, height H2, which is the height of the portions of SiGe regions 410 over the top surface of substrate 320, may be significantly greater than height H1 of SiGe regions 310. For example, height H2 may be about 20 nm, while height H1 may be only about 5 nm, even if SiGe regions 310 and 410 are formed simultaneously. With the great height H2 and the small horizontal sizes, SiGe regions 410 may have pyramid top portions, with the slopes of the top portions being on (111) planes. This creates significant problems for the subsequent process steps such as the formation of source and drain silicide regions.